Titanium-silicalite (TS-1) is a zeolite with transition metal Ti in the framework and MFI framework topology, characterized by good selective oxidation and shape-selectivity. Associated with environmentally-attractive oxidant, aqueous H2O2, TS-1 has been widely applied in selective, catalytic oxidation of organic compounds, such as alcohols, phenols, olefins and ethers, etc. In particular, hydroxylation of phenol, oxidation of cyclohexanone amine and propylene epoxidation have been realized in industrial production.
Since 1981, the synthetic method of TS-1 was firstly published by Macro Taramasso. In the following three decades, hydrothermal synthesis of TS-1 has formed two types through continuous development. One is the adoption of tetrapropylammonium hydroxide (TPAOH) as template to synthesize TS-1 (classic method). The following patents and publications belong to the classic method: U.S. Pat. No. 5,656,252, WO2009077086, CN1167082A, CN 1260241A, CN 1169952A, CN 1239016A, CN1217232A, CN 1239015A, CN1245089A, CN 1247771A, CN1275530A, CN1275529A, CN1294030A, CN1328878A, CN1327947A, CN1418813A, CN1216801C, CN 1488438A, CN 1482062A, CN1634765A, CN 1843626A, CN 1830564A, CN 101134575A, CN101291877A, CN1935651A, CN101190792A, CN101190793A, CN 101434399A, CN101434400A, CN101327934A and CN101696019A, etc.; and Zeolites 12(1992)943-950, Zeolites 16(1996)184-195, Zeolites 19(1997)238-245, Microporous and Mesoporous Materials 22(1998)23-31, Microporous and Mesoporous Material 66(2003)143-156 and Chemical Engineering Journal 147(2009)316-322, etc. Another one is the use of a relatively low price tetrapropylammonium bromide (TPABr) or other inexpensive templates to synthesize the TS-1 system (low-cost method). The following patents and publications belong to the low-cost method: U.S. Pat. No. 5,688,484, CN1167010A, CN 1513760A, CN1806918A, CN 101428814A and CN101767036A, etc.; and Material Chemistry and Physics 47(1997)225-230, Zeolites 19(1997)246-252, Microporous and Mesoporous Materials 12(1997)141-148, Catalysis Today 74(2002)65-75, Applied Catalysis A 185(1999)11 and Chinese Journal of Catalysis 17(1996)173-176, etc.
Besides two above-mentioned hydrothermal synthesis, TS-1 can be synthesized by a variety of methods, such as isomorphous substitution, etc. But owing to the longer bond distance of Ti—O than that of Si—O, it is difficult for Ti atom to be introduced into the framework of zeolite. Therefore, no matter which method used in synthesis of TS-1 would form some non-framework titanium species. The existence of non-framework titanium species would generate two negative effects on the production of TS-1. The first one is these non-framework titanium species have no catalytic activity, but would trigger the decomposition of oxidant hydrogen peroxide. Thus during the reaction it would reduce the catalytic performance of TS-1. The second one is the amount of non-framework titanium species is hard to control, which would results in the different catalytic performance of TS-1 from different synthesis batch.
In order to reduce the bad influence of non-framework titanium species, the following patents and publications are about the modification of TS-1.
U.S. Pat. No. 5,367,099, U.S. Pat. No. 5,607,888, U.S. Pat. No. 5,476,823, U.S. Pat. No. 5,365,003, CN101602013A and CN1844321A introduce a silane modified method for zeolite with MFI topology. A representative patent CN101602013A discloses a silane modified method in gas phase for TS-1, in which the silylation agent under nitrogen atmosphere was introduced into the reaction for 0.5-10 h at 50-300° C.
Patents CN1245090A, U.S. Pat. No. 4,794,198, CN1657168A, CN101591024A and CN101417238A introduce an acid treatment for TS-1. A representative patent CN1657168A discloses an acid treatment for uncalcinated TS-1, in which uncalcinated TS-1 was mixed with acidic solution under room temperature to 200° C., then the regular filtration, wash, drying and calcination were performed.
Patents CN1555923A, CN1268400A, CN101659599A and EP0958861 A1, and publications Catalysis Today 93-95(2004)353-357 and Chemical Engineering (China) Vol39, No1, P53-57 introduce a salt modification for TS-1. A representative patent CN1268400A discloses a modified method by using aqueous solution of a metal salt or mixtures, in which according to the ratio of metal salt:water:zeolite=0.01-10 g:10-100 ml:1 g, TS-1 was added into the aqueous solution of metal salt, and keep the mixture in static state for 6-100 h, then dry it at 30-100° C. by using water bath and further dry it at 110-200° C. in oven for 1-20 h. At last using temperature programmed method to increase temperature from 200 to 800° C. about 1-12 h, the zeolite was calcinated about 2-20 h at this temperature.
The above three modification methods can increase the certain catalytic performance of TS-1. The modification by acid and salt can suppress the negative influence of non-framework of titanium species during the reaction. However, all of these methods cannot essentially eliminate it.
It is reported that inorganic or organic alkalic solution modification of TS-1 can generate holes in TS-1, which is in favor of diffusion of reactants and products.
The following patents introduce modification method of TS-1 by using inorganic or organic alkalic solution.
U.S. Pat. No. 6,475,465B2 and CN1301599A (application date Dec. 24, 1999; application number 99126289.1) both disclose a modification method using organic alkalic solution, in which according to the ratio of organic alkalic solution (such as aliphatic amines, alcohol amines, quaternary ammonium compounds) or mixtures (mol):TS-1 (g):water (mol)=(0.005-0.5):100:(5-200), to make them mixed and reacted under 150-180° C. for 2 hours to 3 days. The TS-1 zeolite used here can be the raw or acidic modified TS-1.
Patent CN124090A (application date Aug. 18, 1998, application number 98117503.1) discloses a further modification method using organic alkalic solution for acidic-modified TS-1 sample, in which the mixture of TS-1 sample and acidic solution reacted for 5 min to 6 h under 5-95° C. Then, the acidic-modified TS-1 sample and organic alkalic solution were mixed, and reacted in sealed reactor for 2 h to 8 days under 120-200° C. and autogenous pressure. The organic alkali used here can be aliphatic amines, alcohols amines, quaternary ammonium compounds, etc., or the mixture of these organic alkali.
Patent CN101850985A (application date Mar. 31, 1998, application number 200910131993.5) discloses a method using alkaline solution of pore former to modify TS-1 sample. In this method, TS-1 sample was added into alkaline solution of pore former, the mixture with the ratio of TS-1:pore former:alkali:water=100:(0.001-5):(0.005-5):(200-10000) was attained. Then the mixture reacted for 2-360 h under 80-200° C. and autogenous pressure. The pore former can be sucrose, starch, furfural, phenol, benzothiophene, dibenzothiophene, naphthyl, quinoline, carbazole, indole, polypropylene, polyethylene glycol, polystyrene, polyvinyl chloride, polyethylene and the mixtures or derivatives of these compounds. The alkaline source can be divided into organic or inorganic alkali, in which organic alkali can be urea, quaternary ammonium hydroxides compounds, aliphatic amines, alcohols amines and the mixture of these compounds; inorganic alkali can be ammonia, sodium hydroxide, potassium hydroxide, barium hydroxide and the mixture of these compounds.
Patent CN101537372A, CN101618338A, CN101618339A, CN101623653A, 101658791A, CN101658798A, CN1016646696A, CN101665256A and CN101670298A disclose a modification method for TS-1 using alkaline solution involving noble metal. In this method, the mixture of TS-1, aqueous solution of silicon, noble metal source, protective agent and alkaline source was hydrothermally reacted in sealed reactor, and recycled the products. The noble metal source can be the oxides, halides, carbonates, nitrates, ammonium salts and hydroxides of Ru, Rh, Pd, Re, Os, Ir, Pt, Ag and Au, or other compounds of these metal. Protective agent can be glucose, cyclodextrin, polybenzimidazole, polypropylene, polyethylene glycol, polystyrene, polyvinyl chloride and polyethylene, etc. Surface active agents include cationic surfactants, anionic surfactants and nonionic surfactants. The alkaline source can be divided into organic or inorganic alkali, in which organic alkali can be urea, quaternary ammonium hydroxides compounds, aliphatic amines, alcohols amines and the mixture of these compounds; inorganic alkali can be ammonia, sodium hydroxide, potassium hydroxide, barium hydroxide and the mixture of these compounds.
Patent CN1260241 (application date Apr. 10, 1998, application number 98101357.0) discloses a modification method using alkaline hydrolysis of titanium source solution. In this method, according to the ratio of hydrolysis of titanium source solution:TS-1 sample=200-1500:1, the mixture was crystalized in reactor for 1-8 days under 120-180° C., then TS-1 with extra Ti was obtained after filtration, wash and drying. The alkaline solution can be quaternary ammonium alkali compounds, aliphatic amines and alcohols amines, or the mixture of these compounds.
Patent CN1421389A (application date Nov. 29, 2001, application Ser. No. 01/140,182.6) disclose a modification method using alkaline solution of silicon. In this method, according to the ratio of aqueous solution of silicon:TS=70-1500:1, the mixture was reacted in reactor for 0.1-150 h under 120-180° C., then silicon-modified TS-1 was obtained after filtration, wash and drying. The alkaline solution can be quaternary ammonium base compounds, aliphatic amines and alcohols amines, or the mixture of these compounds.
Patent CN101850986A (application date Mar. 31, 2009, application number 200910131992.0) disclose a modification method using mixed alkaline solution (organic and inorganic). In this method, according to the ratio of TS-1:inorganic alkalin:organic alkalin:water=100 g:(0.005-5 g):(0.01-10 mol):(200-10000 mol), the mixture was reacted for 2-360 h under 80-200° C. and autogenous pressure. Organic alkali can be urea, quaternary ammonium hydroxides compounds, aliphatic amines, alcohols amines and the mixture of these compounds; inorganic alkali can be ammonia, sodium hydroxide, potassium hydroxide, barium hydroxide and the mixture of these compounds. And the ratio of organic and inorganic alkali is 1-50:1.
The following publications also report a modification method for TS-1 by using organic alkaline solution.
Microporous and Mesoporous Materials 102 (2007) 80-85 reported a modification method by using aqueous solution of tetrapropylammonium hydroxide. In this method, TS-1 sample (1 g) was added into the mixed aqueous solution of 4.17 ml TPAOH (1M) and 3.32 ml water, then crystalized under static condition and 170° C. for 24 h. After filtration, wash and drying, modified TS-1 was calcinated for 16 h under 520° C.
The mater dissertation by Baoji Zhang titled “The Study About Synthesis of TS-1, Alkali Modification, Extrudation and Catalytic oxidation of cyclohexane” reported a modification method by using organic alkaline solution. In this method, the organic alkaline solution included TPAOH, ethanolamine, ammonia, hexamethylene tetramine, tetraethyl ammonium hydroxide, the mixture of ammonia and TPABr, the mixture of tetraethyl ammonium hydroxide and TPABr. It is worth mentioning that the catalytic performance has been improved more than twice by using TPAOH. And the catalytic performance has been nearly doubled by using the mixture of ammonia and TPABr, the mixture of tetraethyl ammonium hydroxide and TPABr.
The mater dissertation by Janbo Yin titled “The Optimization of Styrene Epoxidation with H2O2 on TS-1” reported a modification method for TS-1 by using organic, inorganic alkaline solution and alkaline salts. In this method, TS-1 sample was added into the solution of organic, inorganic and salts to react for 24 h. Then modified TS-1 was obtained after filtration, wash, dry under 100° C. and calcination for 6 h under 540° C. The salt includes Na2CO3, sodium citrate, sodium acetate and NaNO3. The organic alkali includes TPAOH, tetraethyl ammonium bromide (salt), triethanolamine, propylamine and urea. It is worth mentioning that the modification using inorganic alkaline solution and salt was not good as that by using organic alkaline solution. The only effect of salt during the modification was the cation as inhibitors of acidity.
Many public literatures have reported a modification by using organic alkaline solution, such as many mater dissertations “Synthesis and Modification of Titanium Silicalite-1 and its Performance in Ammoxidation of Methyl Ethyl Ketone” by Lizhen Xia, “Characterization of TitaIlium Silicalite-1 modified by organic base and its performance in Ammoxidation of Methyl Ethyl Ketone” by Peng Li, “Oxidative Desulfurization of Sulfide over Titanium Siliealite” by Lixia Zhao, “The characters and catalysis activities of micro-TS-1 modified by several kind of Alkali” by Jingbo Mao, “Effect Factors in Synthesis Process of TS-1 Zeolite” by Yang Liu, “The Effect of Modification on TS-1 and Gas-Phase Epoxidation of Propylene” by Guanghong Liu, “The synthesis of TS-1 and catalytic performance in propylene epoxidation” by Xinxu Liu, and some references, such as Acta Petrolei Sinica 2008 24(1) 57-62, Journal of Fuel Chemistry and Technology 2008 36(4)484-488. In this method, alkaline solution includes TMAOH, TEAOH, TPAOH (best modification effect), TBAOH, NaOH, NH3 and Na2CO3, etc.
To sum up, the effect of inorganic alkaline solution for TS-1 treatment is to dissolve the framework of TS-1, and then internal cavities in TS-1 were generated. General organic bases such as aliphatic amines and alkanolamines have similar performance as inorganic alkali, but quaternary ammonium bases not only can dissolve the framework of zeolite, but also can make dissolved silicon titanium species re-crystalized resulting in some non-framework titanium into framework of zeolite. It is generally thought the effect of treatment of TPAOH for TS-1 is better than that of other quaternary ammonium alkalis. However, the TPAOH treatment has application issue. Besides TS-1 sample synthesized by classic methods and some low-cost methods, not all TS-1 sample can be modified well. The main reasons which cause this phenomenon is big crystal and many amorphous non-framework titanium species in low-cost synthesized TS-1 sample. These two reasons would result in big diffusion resistance, because of longer diffusion path for dissolved titanium species during modification. Furthermore, titanium species in solution is favorable to form TiO2 (anatase). Therefore, the activity of many low-cost synthesized TS-1 after TPAOH modification is not significant improved.